Carbocationic fluorescence and its efficient electron-transfer quenching

Abstract

The absorption and emission spectral properties of several carbocations have been examined in acidified acetonitrile and Nafion matrices. Xanthenium and thioxanthenium carbocations are characterized by moderately strong yellow-to-orange fluorescence (τ_F = 1-30 ns). In contrast, triphenylmethyl carbonation is very weakly emitting. The fluorescence of xanthenium and thioxanthenium carbocations is readily quenched by aromatics by a predominantly charge-transfer mechanism; the quenching rate constants are in the limit of diffusion control. Electron-transfer-derived radical cations (from aromatic quenchers) and radicals (from carbocations) are observed in the course of nanosecond laser flash photolysis. The yields of net electron transfer are, however, low (≤0.02). suggesting efficient back electron transfer in the radical cation/radical pairs in singlet configuration.